The invention relates to a method of manufacturing a semiconductor device whereby field oxide regions are formed in a surface of a silicon body through oxidation, which body is provided with an oxidation mask which is formed in a layered structure provided on the surface and comprising a lower layer of silicon oxide, an intermediate layer of polycrystalline silicon and an upper layer of material comprising silicon nitride, wherein windows are etched into the upper layer, the intermediate layer is removed by etching within the windows and below an edge of the windows, a cavity is formed below said edge, after which material comprising silicon nitride is provided in the cavity.
Such a method is particularly suitable for manufacturing semiconductor devices with field oxide regions of sub-micron dimensions. The field oxide regions can be formed practically without dimensional losses, i.e. they have dimensions which are practically identical to those of a photoresist mask which is used for providing the windows in the upper layer of the oxidation mask. In addition, the occurrence of defects in the silicon body during oxidation is avoided since the lower layer of silicon oxide and the intermediate layer of polycrystalline silicon in the oxidation mask (stress-relief layers) satisfactorily absorb stresses which may arise during the oxidation treatment owing to differences in coefficient of expansion between silicon and the material comprising silicon nitride.
After the field oxide regions have been formed, the oxidation mask is removed. Semiconductor elements are provided in the regions of the silicon body enclosed by the field oxide regions. The former regions are accordingly called "active regions". If these semiconductor elements are MOS transistors, the active regions are first provided with a layer of gate oxide, on which a gate electrode is then provided.
U.S. Pat. No. 4,755,477 discloses a method of the kind mentioned above whereby silicon nitride is provided below the edge. Before this is done, the lower layer of silicon oxide is first etched away inside the windows by means of a usual etchant solution.
The known method has the disadvantage that the gate oxide provided on the active regions enclosed by the field oxide may exhibit defects, so that breakdown of the gate oxide can occur at usual voltages between the gate electrode and the active region.